It is a current tendency to utilize pigments as colorants not only in various printing inks and paints but also in those fields in which clearer colorations are required, for example in pigment dispersion-based resist compositions for color filters and inks for ink jet printing.
In recent years, marked advances have been made in the technology of dispersing such pigments. The method which is used for further rationalization of production processes comprises carrying out dispersion so as to increase the pigment concentration as far as possible. However, there is a fundamental problem that as the pigment concentration increases, the fluidity and dispersion stability decrease.
In addition, in the fields of color filters and ink jet recordings, where high levels of fastness are required, it is necessary to use high-grade pigments and reduce the particle diameter to a finer level so that clearer colorations may be accomplished. However, high-grade pigments are by nature poor in dispersion stability as compared with other pigments and, further, as the particle diameter becomes finer, the tendency toward aggregation increases and it becomes difficult to obtain stable dispersions.
On the other hand, carbon black is widely used as a pigment showing a high level of blackness. However, it has problems, namely difficulty in attaining high concentrations thereof and insufficiency in dispersion stability due to its characteristics that its primary particle diameter is very small and it is porous and has a large specific surface area. Recently, attempts have been made to use carbon black also as a light-shielding material for black matrices. It is a problem, however, that it is difficult to obtain highly insulating black matrices from carbon black which is highly conductive.
Therefore, for solving such problems, investigations have been made concerning, for example, the treatment (surface treatment/modification) of pigments themselves or the development of pigment dispersants, pigment derivatives or surfactants, each having marked ability to wet (adsorption ability) pigments and/or be wetted to increase the pigment dispersion stability and/or pigment concentration, and a number of research works are now still going on.
For example, as a method utilizing acid-based affinity, Japanese Kokai Publication Sho-54-037082 discloses a method of attaining good pigment dispersibility and pigment dispersion stability by utilizing a basic group-containing pigment dispersant for dispersing acidic pigments and thus allowing the pigment dispersant to be adsorbed on the pigment surface through acid-based affinity.
Moreover, a method utilizing affinity attributable to similarity of the molecular structure using pigment derivatives, etc. has also been investigated. For example, in Japanese Kokai Publication 2001-356210, there is disclosed a method of obtaining good dispersion stability by using a phthalocyanine pigment derivative introduced with a sulfonic acid group to make a pigment derivative be adsorbed on the pigment surface having similarity in the molecular structure.
Furthermore, the applicant proposes, as a method utilizing the covalent bonding between a functional group occurring on the pigment surface and a dispersion-treating compound, a method of obtaining good pigment dispersibility by using a carbodiimide group-containing compound for dispersing a pigment having a functional group reactive with a carbodiimide group such as an acid group, amine group and hydroxyl group to make the covalent bonding between said functional groups have higher bonding power in Japanese Kokai Publications 2002-201381 and 2002-186838.
However, in such method utilizing acid-based affinity, an acid group is required to occur, or in such method utilizing the covalent bonding, a functional group reactive with a carbodiimide group is required to occur, on the pigment surface, respectively. Thus, there has been a problem that these methods could not be applied to pigments which do not contain those functional groups.
Furthermore, when the functional group reactive with a carbodiimide group is an amino group or hydroxyl group, the reaction temperature is high, thus there has also been a problem that heating is required at the time of dispersion treatment for pigments having these functional groups.
On the contrary, in the method utilizing affinity attributable to similarity of the molecular structure, although the pigment surface does not require to have a special functional group, there has also been a problem that a large amount of materials must be used for dispersing a pigment.
For example, in the method described in Japanese Kokai Publication 2001-356210, the pigment is dispersed by combinedly using a pigment derivative and pigment dispersant, but when the total amount of the pigment derivative and the pigment dispersant to be used is reduced, preferable pigment dispersibility or dispersion stability cannot be obtained. Thus, there is a major problem that it becomes difficult to correspond to high concentration of the pigment.
In addition, if the pigment derivative containing an acid group occurs in a composition, when the composition is used as a pigment dispersion-based resist composition for color filters and black matrices, there is a problem that the liquid crystal display screen is defaced as a result of bleeding of a pigment derivative during the alkaline development.